This application claims priority of Japanese Patent application number 2000-281927, filed Sep. 18, 2000.
This invention relates to an optical multiplexer to multiplex optical signals having different wavelengths and an optical demultiplexer to demultiplex wavelength multiplexed optical signals.
To realize high-density wavelength multiplexed optical transmission, an optical multiplexer/demultiplexer to multiplex optical signals having different wavelengths and to demultiplex wavelength multiplexed optical signals into individual wavelengths in a low crosstalk. As one means, a combination of an interleaved optical filter to distribute a plurality of optical signals having wavelengths at constant interval into two ports and an arrayed waveguide grating has been studied.
As shown in FIG. 3, an interleaved optical filter consists of multi-stage Mach-Zehnder interferometers connected in serial and has periodic bandpass characteristics and relatively steep loss wavelength characteristics (See H. Arai et al., xe2x80x9cImprovement of Isolation of Interleave Multi/Demultiplexers using three-stage MZIsxe2x80x9d, Proceedings of The 2000 IEICE General Conference, C-3-85, JAPAN, 2000). FIG. 4 is a schematic diagram showing insertion loss wavelength characteristics of an interleaved optical filter to reduce interval of 100 GHz of wavelength multiplexed optical signals into interval of 50 GHz.
However, when the wavelength interval become as narrow as 0.3 nm, it becomes difficult to sufficiently suppress the crosstalk. In addition, an optical filter generally has chromatic dispersion characteristics due to a production error etc., and this causes waveform deterioration owing to frequency chirping. As shown in FIG. 5, assuming that the transfer function between ports X0 and Y0 of an interleaved optical filter is H1, the transfer function between ports X1 and Y1 is H1*, the transfer function between port X0 and Y1 is H2, and the transfer function between ports X1 and Y0 is H2*, those transfer functions show chromatic dispersion characteristics and transmission factor characteristics as shown in FIG. 6.
FIG. 6 is a schematic diagram showing characteristics in one cycle of the straight direction (between the ports X0, Y0 and between the ports X1, Y1) and the cross direction (between the ports X0, Y1 and between the ports X1, Y0). In FIG. 6, the horizontal axis expresses wavelength, and the vertical axis expresses chromatic dispersion and transmission factor. The transmission characteristics of the transfer functions H1 and H1* are equal each other and the transmission characteristics of the transfer functions H2 and H2* are equal each other. However, the chromatic dispersion characteristics of the transfer functions H1 and H1* vary reversely from each other relative to a wavelength, and the chromatic dispersion characteristics of the transfer functions H2 and H2* vary reversely from each other relative to a wavelength. In other words, when the chromatic dispersion characteristics of the transfer functions H1 and H1* are multiplied, a constant value which does not depend on the wavelength is obtained, and similarly, when the chromatic dispersion characteristics of the transfer functions H2 and H2* are multiplied, a constant value which does not depend on the wavelength is obtained. Since the interleaved optical filter satisfies the principle of reciprocity, its transfer function basically does not depend on the propagation direction of the light.
Although it is possible to obtain the steep loss wavelength characteristics, namely the chromatic dispersion characteristics having little crosstalk by increasing the number of the stages of Mach-Zehnder interferometer, it is difficult to produce such an optical filter and also there is a problem that the element length is increased.
Furthermore, because the chromatic dispersion that each wavelength receives is not even, a dispersion compensating element must be newly installed to equalize the chromatic dispersion for each wavelength.
When two interleaved optical multiplexing/demultiplexing elements are connected in serial so that the optical signal propagates passing through both straight and cross connections, it is theoretically possible to flatten the chromatic dispersion characteristics (See H. Arai et al., xe2x80x9cReduction of Chromatic Dispersion of Wavelength Interleaver using three-stage MZIsxe2x80x9d, Proceedings of The 2000 Electronics Society Conference, C-3-15, JAPAN, 2000). However, it is difficult to produce two interleaved optical multiplexing/demultiplexing elements having the identical transmission characteristics, and thus chromatic dispersion depending on a wavelength still remains in a practical situation.
It is therefore an object of the present invention to provide an optical multiplexer and demultiplexer having flat chromatic dispersion characteristics.
An optical multiplexer according to the invention consists of an interleaved optical filter having first and second port pairs, a connector to connect between the two ports composing the second port pair of the interleaved optical filter, a first optical circulator having first, second and third ports to output an input light of the first port from the second port and output an input light of the second port from the third port, a second optical circulator having first, second and third ports to output an input light of the first port from the second port and output an input light of the second port from the third port, and a combiner to combine the output light from the third port of the first optical circulator and the output light from the third port of the second optical circulator.
The second port of the first optical circulator connects to one of the two ports composing the first port pair of the interleaved optical filter, and a first signal light enters the first port of the first optical circulator. The second port of the second optical circulator connects to the other port of the two ports composing the first port pair of the interleaved optical filter, and a second optical signal enters the first port of the second optical circulator.
Also, the optical multiplexer according to the invention further comprising an optical filter having first and second port pairs, a connector to connect between the two ports composing the second port pair of the optical filter, a first optical circulator having first, second and third ports to output an input light of the first port from the second port and output an input light of the second port from the third port, a second optical circulator having first, second and third ports to output an input light of the first port from the second port and output an input light of the second port from the third port, and a combiner to combine the output light from the third port of the first optical circulator and the output light from the third port of the second optical circulator.
The optical filter has the optical transmission characteristics in which the chromatic dispersion characteristics of two transfer functions in straight direction between the first and second port pairs are in the opposite relation from each other relative to a wavelength and the chromatic dispersion characteristics of two transfer functions in cross direction between the first and second port pairs are also in the opposite relation from each other relative to a wavelength, and transmission wavelengths which differ according to the straight and cross propagation between the first and second port pairs.
The second port of the first optical circulator connects to one of the two ports composing the first port pair of the optical filter, and a first optical signal enters the first port. The second port of the second optical circulator connects to the other port of the two ports composing the first port pair, and a second optical signal enters the first port.
An optical demultiplexer according to the invention consists of an optical divider to divide an input light into two portions, an interleaved optical filter having first and second port pairs, an optical isolator to connect between the two ports composing the second port pair of the interleaved optical filter so that it allows optical propagation from one port to the other and refuses the optical propagation in the opposite direction, a first optical circulator having first, second and third ports to output an input light of the first port from the second port and output an input light of the second port from the third port, and a second optical circulator having first, second and third ports to output an input light of the first port from the second port and output of the second port from the third port.
The second port of the first optical circulator connects one of the two ports composing the first port pair of the interleaved optical filter, and one portion of the divided light by the optical divider enters the first port. The second port of the second optical circulator connects to the other port of the two ports composing the first port pair of the interleaved optical filter, and the other portion of the divided light by the optical divider enters the first port.
Also, the optical demultiplexer according to the invention consists of an optical divider to divide an input light into two portions, an optical filter having first and second port pairs, an optical isolator to connect between the two ports composing the second port pair of the optical filter so that it allows optical propagation from one port to the other and refuses the optical propagation in the opposite direction, a first optical circulator having first, second and third ports to output an input light of the first port from the second port and output an input light of the second port from the third port and a second optical circulator having first, second and third ports to output an input light of the first port from the second port and output an input light of the second port from the third port.
The optical filter has the optical transmission characteristics in which the chromatic dispersion characteristics of two transfer functions in straight direction between the first and second port pairs are in the opposite relation from each other relative to a wavelength and the chromatic dispersion characteristics of two transfer functions in cross direction between the first and second port pairs are also in the opposite relation from each other relative to a wavelength, and transmission wavelengths which differ according to the straight or cross propagation at the propagation between the first and second port pairs.
The second port of the first optical circulator connects to one of the two ports composing the first port pair of the optical filter, and one portion of the divided lights by the optical divider enters the first port. The second port of the second optical circulator connects to the other port of the two ports composing the first port pair of the optical filter, and the other portion of the divided lights by the optical divider enters the first port.